This invention relates generally to civil engineering and more particularly to a casting a manhole, that is, a concrete shell that is placed below ground, intersecting a sewer or other conduit, to provide access from above ground.
Because of certain problems which may occur at the point where a sewer pipeline changes direction, or where a junction is formed at the intersection of two or more pipe-lines, manholes are often installed at these locations.
It is necessary, when using a manhole, to insure that sewage flow through the manhole is uninterrupted. Thus, it is common practice to construct a channel or channels in the floor of the manhole connecting the influent and effluent pipe openings in the walls in order to maintain the same rate of flow as that of the sewer pipe. Also, inasmuch as the manhole itself creates an influent source from ground surface water above, and there may be surges in the sewer system, the surface of the floor is usually built with an inward slope which directs water into recessed channels to prevent sewage and debris from collecting on the manhole floor surface. This surface is commonly referred to as the manhole "invert".
Various methods of configuring the floor or invert of the manhole as described above are used. In manholes which are constructed on site, usually of masonry, the invert may be formed by constructing the manhole around a pipe, then breaking out the top portion of the pipe, leaving a trough or channel running through the manhole. However, on-site construction of brick or masonry manholes is time consuming and costly. It has been found that precast concrete sewer manholes are more cost effective, quicker to install, and superior in construction to masonry manholes; therefore, precast manholes are widely used.
it is common practice to manufacture the floor surface of the base section of the precast manhole generally flat and level, perhaps because that is easier. Influent and effluent openings are cast into-the wall at a specified distance above the flat floor surface, to permit the invert channel and sloped floor surface to be constructed in the field once the manhole is in place. This practice, like that used in on-site masonry manhole construction adds labor costs to the manhole and does not eliminate the problems associated with the transportation of materials and equipment to the jobsite.
It would be most desirable for the precast manhole to be produced in such a way that the invert configuration of channels and sloped floor were formed as an integral portion of the base section of the manhole, thus leaving no additional construction to be performed in the field.
Besides the above-described methods of forming a manhole invert in the field, at least one process exists wherein recessed channels are cast into the floor surface as the base section is cast. This process, known as the "Morebase Invert System", is understood to employ a recessed pouring plate which is substituted for the flat pouring plate located over the core of the form used to produce the manhole base section. Various U-shaped channel forms are supplied to accommodate various angles of intersection of pipes into the manhole and are attached to the pouring plate by means of strong magnets. These channel forms are positioned such that they align and abut the forms used to produce the pipe entrance openings in the walls of the manhole.
A number of problems arise in the Morebase system, related both to the manufacturing process as well as the finished product. With respect to manufacture, the channel forms, although held in place by strong magnets, may be dislodged when concrete is poured into the manhole form. Further, a tight fit between each end of the channel form and the corresponding hole form to which it abuts is unlikely, thus giving rise to the possibility of concrete paste entering the void. The resulting flash must be removed in order to open the hole after the product is cured. These manufacturing problems thus detract from product quality, and require labor for correction. Additionally, this design produces a floor surface with substantially flat surfaces that do not drain properly. Thus, the "Morebase" invert forming system, while attempting to eliminate the problems associated with field construction of manhole inverts may be improved upon.